The universal biological donor for selenium incorporation into proteins has been identified as monoselenophosphate. Selenophosphate synthetase catalyzes the formation of this reactive selenium donor compound from selenide and ATP. In recent years, the catalytic mechanism of the selenophosphate synthetase enzyme from Escherichia coli has been studied extensively. Mutagenic studies have demonstrated that a cysteine residue (cys-17) is essential for catalytic activity of the enzyme. Recently selenophosphate synthetase genes have been identified in other organisms. The enzyme from Methanococcus jannaschii has been identified from the published genome sequence and was found to contain a selenocysteine residue at position 17. Through a collaboration with Kevin Sowers, we have obtained M. jannaschii bacteria grown in media containing 75Se. Cell extracts have been prepared and 75Se selenophosphate synthetase will be purified to homogeneity. At the same time the human selenophosphate synthetase enzyme will be expressed and purified from an expression vector provided by Marla Berry. This enzyme contains a threonine at position 17. Both purified enzymes will be kinetically characterized and the activity will be compared to the E. coli protein.